I grew up in the Philadelphia area and spent my early career there. I thought I knew most places in the area, so when I heard about it's Graffiti Underground, I was perplexed that I never heard of it. Turns out that it is an abandoned pier jutting out into the Delaware River just a few minutes north of downtown. You have to know where it is in order to find it as there are no signs.
My first impression of it was that it was pretty big and colorful (it is also used by paintballers). It seemed that every direction I looked, there were great leading lines creating very cool vanishing points like the one in this image. The light was overcast and from the north on this shot. I am sure on a sunny day, there would be some more interesting plays of light and shadows. I will have to head back to see if I am right.